Think about your last road trip around time. Maybe a day driving up the hill to a ski resort. Or pulling your boat to a reservoir for a day of water skiing. Biking more your style? There are trails all over – your car can help you get there. 

But here in Denver, protecting that air means following local emissions regulations. And if your car’s due for its emissions test, you might be wondering: What could go wrong?

Your car’s passed before. What if it doesn’t? 

Whether you’re preparing for your first emissions inspection or trying to figure out why your car didn’t pass, it helps to know the most common reasons for failure.

We see it all the time in our shop, and trust us—most of the time, it’s fixable. Let’s walk through the top five reasons cars fail emissions tests in Denver (and how to keep your car on the right side of the law—and the environment).

Check Engine Light Is On

This one’s a no-brainer. If your check engine light is glowing, your car is going to fail the test. No exceptions.

Why? Because a lit check engine light usually means your car’s onboard diagnostic system (OBD-II) has detected an issue—often related to emissions. That light might signal a misfire, faulty oxygen sensor, loose gas cap, or something more serious.

Luckily, there are many things you can do:

• Don’t ignore it. Even if your car seems to run fine, the issue could be affecting your emissions output.
• Use a diagnostic scanner to pull the trouble code, or swing by our shop, and we’ll check it for you.
• Fix the underlying issue. Simply clearing the code without fixing the problem won’t help. Your car will fail again.

If your light is off but was recently reset, your vehicle’s onboard monitors may still be in “not ready” mode, which can also result in a failed test.

Faulty or Worn-Out Oxygen Sensors

Your car’s oxygen sensors (O2 sensors) are critical to maintaining proper air-fuel ratios in your engine. When they start to fail, your car burns fuel less efficiently—leading to higher emissions and a failed test.

In Denver’s emissions tests, these sensors are one of the most commonly flagged components. How do you tell if your oxygen sensor might be the issue?

• Decreased fuel economy
• Rough idling or engine hesitation
• Check engine light triggered with O2-related codes

To avoid this, replace worn-out sensors around the 60,000 to 100,000-mile mark. Don’t wait until your fuel efficiency drops—preventative replacement is often cheaper than post-failure repair.

If you’re not sure whether your sensors are in good shape, we can run a quick diagnostic to check their function before you head to your emissions test.

Evaporative Emissions Control System (EVAP) Leaks

Your EVAP system captures fuel vapors from your gas tank and keeps them from escaping into the atmosphere. If there’s a leak or malfunction—such as a cracked hose, damaged charcoal canister, or even just a loose gas cap—it can cause your vehicle to fail. Common causes of EVAP failures include:

• A loose or missing gas cap (yes, really!)
• Cracked vacuum hoses
• Faulty purge or vent valves

To fix these, check your gas cap. Make sure it’s tightened until it clicks—and that the seal isn’t cracked or worn. Have the EVAP system smoke-tested for leaks. And replace any faulty components.

A damaged EVAP system can cause increased hydrocarbon emissions—something Denver inspectors are on the lookout for. Catching this early can prevent a headache later.

Bad Catalytic Converter

Your catalytic converter is at the center of your car’s emission system. It turns harmful pollutants like carbon monoxide and nitrogen oxides into less harmful gases before they exit your exhaust pipe. If it fails, your emissions skyrocket—and you’ll almost certainly flunk the test. You’ll know your catalytic converter might be failing if you experience:

• Sluggish engine performance
• Rotten egg smell from the exhaust
• Loud rattling sound underneath your car
• A failed emissions test with high NOx, CO, or HC readings

If the converter is clogged or damaged, it usually needs to be replaced. Keep up with routine maintenance like oil changes and spark plug replacements—bad combustion can shorten the lifespan of your catalytic converter.

In Denver, catalytic converter failure is one of the top reasons for test failure—especially on older vehicles. Maintenance is the key to staying ahead of potential failures. 

Dirty or Worn-Out Air Filter and Fuel System Issues

Sometimes, failing your emissions test comes down to something simple: your engine can’t breathe properly. A dirty air filter can mess with the air-fuel mixture, making your engine run rich (too much fuel, not enough air). That increases hydrocarbon emissions and triggers diagnostic trouble codes.

Add to that:

• Clogged fuel injectors
• A faulty mass airflow sensor (MAF)
• Bad spark plugs or wires

…and your car may run poorly and emit too many pollutants to pass inspection.

Here’s what you can do:

• Replace your air filter every 12,000 to 15,000 miles.
• Clean or replace the MAF sensor if it’s dirty or malfunctioning.
• Keep your fuel system clean with high-quality gasoline and regular fuel system cleanings.

Routine maintenance on these components can significantly lower your vehicle’s emissions—and help it run better in the long run.

What If Your Car Fails the Emissions Test?

Don’t panic. A failed test doesn’t mean the end of your driving privileges. In most cases, you’ll get a printout with the failure codes and a grace period to make necessary repairs and retest.

Here’s what we recommend:

• Bring your test results to our shop—we’ll review them with you.
• Let us perform a diagnostic inspection to confirm the issue.
• We’ll make the necessary repairs and ensure everything’s emissions-compliant.
• Then, we’ll let you know when your car is ready for a retest.

At Denver Express Care, we’re experts in emissions repairs and compliance. We understand how Colorado’s Air Care Colorado program works and can help you meet the requirements with as little hassle as possible.

Keeping Denver Clean—And Your Car On The Road

Denver drivers face unique challenges: altitude, changing seasons, tough terrain, and tight emissions rules. Staying compliant doesn’t just mean passing your test—it means helping keep our skies clean and our air healthy.

The good news? You don’t have to face it alone.

At Denver Express Care, we’ll help you understand why your car failed, how to fix it, and how to avoid issues in the future. Whether it’s a faulty sensor, clogged catalytic converter, or just a loose gas cap, we’ve seen it—and we know how to fix it.

If your emissions test is coming up—or if you’ve already failed—give us a call or stop by. We’ll get you back on the road quickly, affordably, and with confidence.

Because here in Denver, clean air starts with a well-running car. And we’re here to help you every step of the way.